Interaction of damage from nonionizing and ionizing radiations has been studied in a number of biological systems, but most extensively in microorganisms. Synergistic action has been shown in cell killing and in induction of mutations. In Chinese hamster cells, however, the damage from the two radiations is not synergistic. When doses of each radiation producing equal levels of survival are given immediately after each other in the middle of the DNA synthetic phase, where the greatest interaction takes place, cell killing from X-ray damage is completely additive to the far-UV sublethal damage. However, far-UV damage is only in part additive to sublethal damage induced by X-rays. When an interval for repair is introduced between the two radiations, the loss of interaction was more rapid after an initial dose of X-rays than after an initial dose of far-UV. These observations could be relevant to other end points like mutation and neoplastic transformation. We propose to use mammalian cells in culture, Chinese hamster and mouse embryo C3H/10T1/2 cells, to determine the modifications in the frequency of mutations and neoplastic transformation due to far-UV and X-ray damage interaction. The effect of dose fractionation of single and combined treatments will be determined, and the effects obtained will be analyzed relative to the known repair processes in mammalian cells. If DNA damage is involved in induction of mutation and/or neoplastic transformation in a way similar to the DNA damage leading to cell lethality, combined treatments with cytotoxic agents may yield frequencies different from those predictable on the basis of independent action.